1. Field
Embodiments of the present invention generally relate to high frequency filters, and in particular magnetic filters utilizing thin films in a microstrip device.
2. Description of the Related Art
This invention is primarily directed to communications using frequencies in the 5-100 GHz range. This area encompasses the higher frequencies associated with the microwave range, and the lower frequencies associated with the millimeter range. This range of the spectrum is currently being used, but the current uses are not taking full advantage of this resource. This under utilization exists for a variety of reasons, related both to policy and technology. Limitations in the component technology are a critical obstacle to better utilization of the higher spectra. Many of these technical problems have been or will soon be solved. The novel approach of this invention is one such advancement, and could lead to far better utilization of the frequencies at issue.
The growing interest in this area of the spectrum comes from two important factors. First, the radio and lower frequency microwave portions of the spectrum (i.e. lower frequencies) are significantly overcrowded. Second, the optical/infrared portions of the spectrum (i.e. higher frequencies) suffer significant absorption problems with fog, dust, smoke, and other atmospheric attenuation. The 5-100 GHz range thus occupies something of a sweet spot between these areas. There are other important advantages as well. Small wavelengths enable smaller components, and the high frequencies can provide very high information rate capabilities. However, such waves are not as “robust” as the radio and lower frequency microwave portions of the spectrum, suffering certain attenuation and penetration issues.
Modern communication systems that operate in the 5-100 GHz range, especially in satellite and mobile communications, require high performance filters with low insertion loss and high selectivity. Often, these criteria are fulfilled using a waveguide cavity filter or a dielectric resonator loaded cavity filter because of their low loss capabilities. However, these solutions suffer from excessive size, weight, and cost. To reduce size and cost, and improve reliability, there has been an increasing interest in planar structures.
In recent years, there has been significant progress in many areas of high frequency semiconductor electronics, and a strong movement toward the synthesis of different electronic components into integrated circuits. Initial research into filters suitable for higher frequency ranges focused largely on yttrium-iron-garnet (YIG) in physically large structures. Research has recently been expanded into magnetic MMIC (Microwave-Monolithic Integrated Circuit), using additional materials as well. The operational frequency ƒ can be estimated from the ferromagnetic resonance condition (alternatively referred to as “FMR”), and is set by material properties, such as saturation magnetization Ms, anisotropy fields Hα, the gyromagnetic ratio γ, and the magnitude of an applied field H. If the applied field is along the easy axis, the frequency is given byƒ=γ√{square root over ((H+Hα)(H+Hα+4πMS))}{square root over ((H+Hα)(H+Hα+4πMS))},and therefore the resonance frequency can be varied with an external magnetic field.
This initial research showed that there was promise in thin film magnetic structures capable of operating at higher frequencies. It also illustrated that tunability of operating frequency was possible with a change in the magnitude or orientation of an external magnetic bias. However, this research led to devices which suffered from certain limitations. YIG-based applications have relatively low resonance frequencies, and thus require large external fields to be applied in order to operate above 10 GHz, and very high external fields to operate above 20 GHz. Such large fields are incompatible with devices of a limited size since substantial electromagnets are required.
The disadvantage of YIG-based devices can be overcome with certain magnetic thin film filters that have a much higher internal field, and thus a higher operational frequency. For example, Fe has a much higher resonance frequency for the same applied field. However, its conductivity can lead to high loss at microwave frequencies. Previous work illustrates that structures utilizing thin Fe films can minimize conduction loss while still producing attenuation at certain frequency ranges. However, the maximum attenuation usually reached only about 4-5 dB/cm. This previous work was mostly limited to notch filters, and typically utilized only one layer or type of active material in each device.
Information relevant to attempts to address these problems can be found in the following Publications:    E. Schloemann, R. Tuistison, J. Weissman, H. J. Van Hook, and T. Varitimos, “Epitaxial Fe films on GaAs for hybrid semiconductor-magnetic memories,” J. Appl. Phys. 63, 3140 (1988).    S. Liau, T. Wong, W. Stacy, S. Ali, and E. Schloemann, “Tunable Band-Stop Filter Based on Epitaxial Fe Film on GaAs,” Proc. IEEE MTT-S IMS, 957 (1991).    J. Su, C. S. Tsai, and C. C. Lee, “Determination of Magnetic Properties of Ultrathin Iron Films Using Microwave Stripline Technique,” J. Appl. Phys. 87, 5968 (2000).    N. Cramer, D. Lucic, R. E. Camley, and Z. Celinski, “High Attenuation Tunable Microwave Notch Filters Utilizing Ferromagnetic Resonance,” J. Appl. Phys. 87, 6911 (2000).    A. L. Adenot, O. Acher, T. Taffary, P. Queffelec, and G. Tanne, “Tuneable Microstrip Device Controlled by a Weak Magnetic Field Using Ferromagnetic Laminations,” J. Appl. Phys., 87 6914 (2000).    N. Cramer, D. Lucic, D. Walker, R. E. Camley, and Z. Celinski, “Incorporation of ferromagnetic metallic films in planar transmission lines for microwave device applications,” IEEE Trans. Magn., 37, 2392 (2001)    E. Salahun, G. Tanne, P. Queffelec, M. Le Floch, A. L. Adenot and O. Acher, “Application Of Ferromagnetic Composite In Different Planar Tunable Microwave Devices,” Microwave and Optical Technology Letters, 30, 272 (2001).    C. Lee, W. Wu, C. Tsai, “Ferromagnetic resonance and microstructural studies of Ag/Fe—GaAs waveguide structures,” J. Appl. Phys., 91, 9255 (2002).    E. Salahun, P. Queffelec, G. Tanne, A. L. Adenot and O. Acher, “Tunable Microstrip Stop-Band Function Using Absorption in Layered Ferromagnetic/Dielectric Material”, J. Appl. Phys., 91, 5449, (2002).    Y. Zhuang, B. Rejaei, E. Boellaard, M. Vroubel, and J. N. Burghartz, “GHz Bandstop Microstrip Filter Using Patterned Ni78Fe22 Ferromagnetic Film,” IEEE Microwave Wireless Components Lett., 12, 473 (2002).However, each one of the cited references suffers from at least one of the following disadvantages: excessive size, excessive cost, limited functionality, fabrication difficulties.
For the foregoing reasons, there is a need for high frequency magnetic MMIC filters that provide broader functionality and can still be manufactured on a very small scale using largely conventional fabrication techniques.